Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system.
As shown in FIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG) 100, which includes a biocompatible device case 30 formed of a conductive material such as titanium for example. The case 30 typically holds the circuitry and battery 26 necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 100 includes one or more electrode arrays (two such arrays 102 and 104 are shown), each containing several electrodes 106. The electrodes 106 are carried on a flexible body 108, which also houses the individual electrode leads 112 and 114 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on array 102, labeled E1-E8, and eight electrodes on array 104, labeled E9-E16, although the number of arrays and electrodes is application specific and therefore can vary. The arrays 102, 104 couple to the IPG 100 using lead connectors 38a and 38b, which are fixed in a non-conductive header material 36, which can comprise an epoxy for example.
As shown in cross-section in FIG. 2A, the IPG 100 typically includes an electronic substrate assembly including a printed circuit board (PCB) 16, along with various electronic components 20, such as microprocessors, integrated circuits, and capacitors mounted to the PCB 16. Two coils (more generally, antennas) are generally present in the IPG 100: a telemetry coil 13 used to transmit/receive data to/from an external controller 12; and a charging coil 18 for charging or recharging the IPG's battery 26 using an external charger (not shown). The telemetry coil 13 can be internal to the case 30 as shown, or can alternatively be placed in the header 36.
As just noted, an external controller (EC) 12 is used to wirelessly send data to and receive data from the IPG 100. For example, the EC 12 can send programming data to the IPG 100 to dictate the therapy the IPG 100 will provide to the patient. Also, the EC 12 can act as a receiver of data from the IPG 100, such as various data reporting on the IPG's status. The EC 12, like the IPG 100, also contains a PCB 70 on which electronic components 72 are placed to control operation of the EC 12. A user interface 74 similar to that used for a computer, cell phone, or other hand held electronic device, and including touchable buttons and a display for example, allows a patient or clinician to operate the EC 12. The communication of data to and from the EC 12 is enabled by a coil (antenna) 17.
Wireless data telemetry between the EC 12 and the IPG 100 takes place via inductive coupling, and specifically magnetic inductive coupling. To implement such functionality, both the IPG 100 and the EC 12 have coils 13 and 17 which act together as a pair. When data is to be sent from the EC 12 to the IPG 100 for example, coil 17 is energized with an alternating current (AC). Such energizing of the coil 17 to transfer data can occur using a well-known Frequency Shift Keying (FSK) protocol for example. See, e.g., U.S. Patent Publication 2009/0024179. Inductive transmission of data can occur transcutaneously, i.e., through the patient's tissue 25, making it particularly useful in a medical implantable device system. During the transmission of data, the coils 17 and 13 preferably lie in planes that are parallel, along collinear axes, and with the coils as close as possible to each other, as is generally shown in FIG. 2A. Such an orientation between the coils 17 and 13 will generally improve the coupling between them, but deviation from ideal orientations can still result in suitably reliable data transfer.
FIG. 3 shows further details of the telemetry circuitry in the EC 12 and the IPG 100. Although the telemetry circuitry in these devices may have significant differences in a real application, for the purpose of FIG. 3 such circuitry is illustrated simply, and can be viewed as essentially the same in both devices.
As shown, the EC 12 contains a transmitter TX1 for transmitting a serial stream of digital data (DATA_TX1) to the IPG 100. The transmitter TX1 contains well-known modulation circuitry to modulate the data to an appropriate frequency in accordance with the FSK protocol used. The modulated data is presented to the resonant circuitry (or “tank circuitry”) in the external controller, which consists of an L-C circuit made up of a capacitor and the inductance of the telemetry coil 17. The resonant circuit can comprise a parallel or series connection between the capacitance and the inductance. In one example, a series L-C connection is used when a device is transmitting data, while a parallel L-C connection is used when the device is receiving data. Such dual-mode telemetry circuitry, and rationales for switching between a series and parallel connection depending on whether the circuit is transmitting or receiving, can be found in U.S. Patent Publication 2009/0281597, which is incorporated herein by reference. This Publication is assumed familiar to the reader, and hence its details are not reflected in the circuitry of FIG. 3; instead, the capacitor is shown in dotted lines both in series and in parallel with the inductance of the coil, denoting that it could be in either orientation depending on the circumstances.
The modulated data is presented to the resonant circuitry as an output voltage, Vo1, which in reality comprises rapidly alternating +Vo1 and −Vo1 voltages to provide resonance at the desired FSK frequencies. Stimulating the resonant circuitry in this fashion creates a modulated AC magnetic field, which field is then sensed by at the telemetry coil 13 in the IPG 100. Specifically, the magnetic field induces a current in the coil 13, which ultimately forms a voltage Vi2 across the resonant L-C circuit in IPG 100. Again, the polarity of Vi2 alternates depending on the frequency of transmission. This input voltage Vi2 is presented to a receiver RX2, where it is demodulated to recover the digital data stream (DATA_RX2).
Data transmission from the IPG 100 to the external controller 12 occurs in much the same manner. Digital data (DATA_TX2) is modulated in a transmitter TX2 in the IPG 100, which modulates the data to particular FSK frequencies. The resulting output voltage Vo2 is presented across the resonant circuit formed by a capacitor and the inductance of the telemetry coil 13. The resulting magnetic field is sensed at the telemetry coil 17 in the external controller, and the resulting input voltage Vi1 that forms across the resonant circuit is demodulated to recover the digital data stream (DATA_RX1).
Both of the telemetry coils 17 and 13 are roughly circular, as can be seen in FIGS. 2A and 2B, and each is generally comprised of several windings of wire. As such, each coil 17 and 13 is characterized by an area (A, the area encompassed by the coil) and a number of turns (N, the number of turns of the copper wire used to form the coils). The telemetry circuitry in each device is further characterized by its transmitter output voltage Vo, and its minimum receiver input voltage Vi. This disclosure addresses optimization of such parameters to improve communication distance and reliability between the external controller and the IPG, or between other devices in the implantable medical device system that will be intruded later.